Domain Alias - 2 domains alike/2 mailboxes alike

Question:
1. If we have, say animalshop.tld and animalshopping.tld set up because we want to ensure people finding our shop, and then want to set up a mailaccount which works for both the domains - is this what domain alias is use for?
Because, what I normally whould do was to set up say [email protected] as the "Mailbox" to use, and then set up a "Email Forward" from [email protected] --> [email protected]

2. Setting the "Email Domain" with SpamFilter "Normal" - will be a standard setting for all mailboxes for this domain. So if you set "SpamFilter" in "Email Domain" you do not need to set "spamFilter" in the mailbox unless you want an other setting. Is this correct?

3. Regarding "Domain Alias" Is this a bug? If we setup a "Email Domain" animalshop.tld and animalshopping.tld and then a "Domain Alias" from animalshop.tld --> animalshopping.tld. Then we create the "Mailbox [email protected].. If someone sends a mail to [email protected] (has no mailbox-only domain alias) it will arrive at [email protected] as it should!!
But, if we then make the [email protected] a "Email Catchall" shouldn't the mail to hullahop@animalshopping arrive at [email protected] ??? If not, shouldn't we be able to create the "Email Catchall" animalshopping.tld --> [email protected]?

1. If we have, say animalshop.tld and animalshopping.tld set up because we want to ensure people finding our shop, and then want to set up a mailaccount which works for both the domains - is this what domain alias is use for?

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Alias domains are used to have one website with multiple addresses. It does not impact email.